INTERNATIONAL RESEARCH EXPERIENCE: INVESTIGATING THE SPATIAL VARIATION IN WATER CHEMISTRY OF THE NEWLY FORMED LAKE NGAMI, NORTHWEST BOTSWANA
One of the projects involved investigating the water chemistry of Lake Ngami. Lake Ngami is located in the southeast distal region of the Okavango Delta. The Okavango Delta is a pristine wetland that covers 22,000 km2 in semi-arid northwestern Botswana, Africa.
Lake Ngami is an endorheic system that was dry until 2010 but is now refilling with water. This filling lake provided the unique opportunity to document baseline chemistry that can be used to assess future temporal and spatial trends. The chemistry of the lake was investigated from the mouth to the distal end along a longitudinal transect. Some parameters measured include temperature, total dissolved solids (TDS), and major ion concentrations. Solute concentration (e.g., TDS) increased continuously from the mouth to the distal end of the lake. The solute concentrations are segmented into three distinct regions along the overall increasing trend which could indicate the influx of water from past flood events from 2010 to 2012. This is consistent with the fact that the lake receives inflow of water during an annual flood event and there is a distinct chemical signature of each year’s flood. The highest solute concentrations were observed in the area located at the distal end of the lake. This could be attributed to evaporation or the dissolution of salts within the dry lake bed during the initial reflooding event. The mid-section of the lake has the next highest concentrations of solutes, followed by the section near the mouth. This could be explained by differences in residence time and evaporation effects.
Investigation of the chemistry in the Lake Ngami longitudinal profile shows that (1) the different annual recharge from flooding can be traced through the system and the mixing of water from different recharge periods is slow, and (2) differential evaporation of lake water may affect the solute chemistry.